You can’t go wrong with ACE Group Fitness Instructor certification. But it is what you do after (and maybe before) you get your cert that is important. Learn, learn, learn. Take as many classes taught by different instructors ever chance you get. You might not actually workout in every one of the classes (you can only work out so much), but you can observe and learn. If there is a very popular instructor in your area, go to lots of their classes and figure out why they are so popular. Be a student of fitness and you will be a great instructor.
I allowed my AFAA to lapse because I had a hard time getting CEUs from them. Since they require at least one course from AFAA as part of the renewal, and AFAA never travelled to my area, I had to do their online training in order to renew, and I wasn’t super-happy with the online training I purchased. So I held the cert for 7 years (one late renewal, so I only count 7 years of the 8 year period) and gave it up.
I’ve been very happy with ACE. Easier to get renewal credits.